Editing Helium (section)

====Helium I====
Below its [[boiling point]] of {{convert|4.22|K|C F}} and above the [[lambda point]] of {{convert|2.1768|K|C F}}, the [[isotope]] helium-4 exists in a normal colorless liquid state, called ''helium&nbsp;I''.<ref name="enc" /> Like other [[cryogenic]] liquids, helium&nbsp;I boils when it is heated and contracts when its temperature is lowered. Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered further. <!-- clarifyme / The rate of expansion decreases below the lambda point until about 1&nbsp;K is reached; at which point expansion completely stops and helium&nbsp;I starts to contract again. / if it is below the lambda point, should not it be helium II?-->

Helium&nbsp;I has a gas-like [[index of refraction]] of 1.026 which makes its surface so hard to see that floats of [[Expanded polystyrene|Styrofoam]] are often used to show where the surface is.<ref name="enc" /> This colorless liquid has a very low [[viscosity]] and a density of 0.145–0.125 g/mL (between about 0 and 4 K),<ref name="crc6120">{{RubberBible86th|page=6-120}}</ref> which is only one-fourth the value expected from [[classical physics]].<ref name="enc" /> [[Quantum mechanics]] is needed to explain this property and thus both states of liquid helium (helium I and helium II) are called ''quantum fluids'', meaning they display atomic properties on a macroscopic scale. This may be an effect of its boiling point being so close to absolute zero, preventing random molecular motion ([[thermal energy]]) from masking the atomic properties.<ref name="enc" />